In digital communication systems, intelligence is generally conveyed in the form of sequential pulse codes. The transmission of an audio signal over a digital facility requires that the audio signal be sampled at a rate greater than twice the highest frequency to be transmitted. All frequency components of the audio signal and noise above one-half the sampling frequency must be suppressed prior to sampling. Otherwise, the modulation introduced by the sampling process results in a folded audio signal spectrum which appears as interference in the frequency range below one-half the sampling frequency. This effect, generally known as aliasing, is prevented by accurately controlling the passband of the audio signal prior to sampling.
While aliasing only occurs at transmitting terminals of a digital communication system where an audio signal is sampled, similar passband control arrangements are needed at receiving terminals of the digital facility. Otherwise, the modulation introduced by sampling results in an interferring audio signal above one-half the sampling frequency being applied to the receiving transducer. Electrical filters designed to prevent interference in digital sampling systems are complex and expensive since they must provide accurate filtering over the entire audio frequency range and must be inserted in each transmitting and receiving terminal of the communication system.
Acoustical filter networks have been incorporated in electroacoustic transducer assemblies to control the frequency characteristics of sound waves at the transducer. One such arrangement, disclosed in U.S. Pat. No. 3,819,879 issued June 25, 1974 to Werner Baechtold, provides a low pass acoustical filter integral with a telephone handset receiver cover. The cover is shaped so that sound waves from the transducer are directed through a pair of cavities dimensioned to attenuate higher frequency components. A dimensioned to attenuate higher frequency components. A Helmholtz resonator is incorporated in the receiver cover to provide absorption of sound at its resonant frequency. The Helmholz cavity resonant frequency is set just below one-half the sampling frequency of the digital facility to which the transducer is connected. In this way, the filtering required for digital communication of voice signals is achieved in a more economic manner.
The Baechtold scheme as well as other prior art acoustic filter networks (e.g., U.S. Pat. No. 3,452,164 issued June 24, 1969 to Kiyoshi Kobara) are adaped to modify the frequency characteristics of sound waves travelling along the axis of the acoustic network associated with a telephone receiver. Such arrangements are practical as long as structural dimensions of the acoustic network are small in comparison with the wavelenghts of the sound passing therethrough. When the sound wavelengths are comparable to the structural dimensions, as is generally the case in telephone type transducers and loudspeakers, the higher frequency sound waves cause resonance in the acoustic network. Consequently the unwanted sound wave frequencies are not adequately attenuated by the acoustic filter.
In analog communication systems, higher frequency sound waves outside the desired passband cause little difficulty since they are readily removed in the system. In digital communication systems utilizing sampling, however, the sound wave components of frequency greater than one-half the sampling frequency whose wavelengths are comparable to the acoustic network dimensions partially pass through the network. As aforementioned, these unwanted components are folded into the desired signal frequency band and cause serious interference at transmitter terminals or are passed through the acoustic network associated with the receiving transducer at receiver terminals.